1. Field of the Invention
This invention relates to a method of making aqueous emulsions of crosslinked polydiorganosiloxanes which yield reinforced elastomers.
2. Background Information
A method of polymerizing siloxanes and silcarbanes in emulsion by using a surface active sulfonic acid catalyst is disclosed by Findlay and Weyenberg in U.S. Pat. No. 3,294,725, issued Dec. 27, 1966. Their method, which comprises polymerizing and copolymerizing at least one member selected from organosiloxanes of the unit formula R.sub.n SiO.sub.4-n/2 and silcarbanes having the general formula HO(R).sub.2 SiQSi(R).sub.2 OH in an aqueous medium while in the dispersed state, in the presence of a compound of the formula R'C.sub.6 H.sub.4 SO.sub.3 H as the primary catalyst for the polymerization until the desired increase in molecular aggregation is obtained. These emulsions are stated to be characterized by extreme stability and extremely fine particle size. The products produced were higher molecular weight fluids or solids. In the particular embodiment in which the starting siloxane has a formula ##STR2## it is stated that the product, after neutralization, is a polysiloxane which does not appear gelled but when removed from the emulsion, does gel to a crosslinked rubber. Fillers can be added to the emulsion so that the strength of the rubber resulting from coagulation of the emulsion is further improved.
Axon discloses a method of polymerizing siloxanes and silcarbanes in emulsion employing organic sulfates of the general formula R'OSO.sub.2 OH in U.S. Pat. No. 3,360,491, issued Dec. 26, 1967. His method comprises polymerizing and copolymerizing at least one member of the group consisting of organosiloxanes of the unit formula R.sub.n SiO.sub.4-n/2 and silcarbanes having the general formula HO(R).sub.2 SiQSi(R).sub.2 OH by polymerizing in an aqueous medium while in a dispersed state in the presence of a compound of the formula R'OSO.sub.2 OH until the desired increase in molecular aggregation is obtained. The emulsions are stated as being suitable for release agents and for coating compositions. His embodiment prepared from an alkylalkoxysiloxy endblocked diorganosiloxane is stated as polymerizing to a polysiloxane which does not appear to be gelled, but which when removed from the emulsion, does gel to a crosslinked rubber. Fillers can be added to the emulsion to improve the strength of the rubber resulting from the coagulation of the emulsion. The reinforced emulsion system provides an excellent method of obtaining coatings of tough rubbery siloxane films for release coatings.
A method for emulsion polymerization of organosiloxanes is disclosed by Ikoma in U.S. Pat. No. 3,697,469, issued Oct. 10, 1972. His method emulsifies, in water containing a salt-type anionic surface active agent, an organosiloxane having the unit formula R.sub.a SiO.sub.4-a/2 and then contacting said emulsion with an acid-type cationic exchange resin. The ion exchange converts the salt-type surface active agent to an acid-type thereby starting polymerization of the organosiloxane by making the emulsion an acid medium with a pH value of less than 4. The method is shown to polymerize organocyclosiloxanes, polysiloxane fluids, mixtures of organocyclosiloxanes and alkylalkoxysilanes, mixtures of organocyclosiloxanes and polysiloxane fluids, and alkylalkoxysilanes to give emulsions of polysiloxanes of increased viscosity. The emulsions are useful as coatings to give release properties, lubricants for textiles, and water repellent for textiles. An example combining an organocyclosiloxane and alkyltrialkoxysilane and polymerizing, then mixing the polymerized emulsion with a 10 percent sol of fine silica particles and dibutyltindioctoate emulsion gave a sheet upon drying which was a rubber.
A method of preparing an electrically conductive silicone emulsion is disclosed by Huebner and Meddaugh in U.S. Pat. No. 3,706,695, issued Dec. 19, 1972. The method dissolves a surface active sulfonic acid in water, mixes in a siloxane fluid and homogenizes the mixture to provide a stable dispersion. The dispersion is heated for at least one hour to polymerize the siloxanes, then adds a nonionic emulsifying agent and neutralizes the acid to give a pH of 6.5 to 9. Finely divided carbon black, a metallic salt of a carboxylic acid and a silane of the formula RSi(OR').sub.3 are then mixed into the emulsion. When the emulsion is applied to a substrate and dried, a heat stable electrically conductive silicone rubber is formed. Satisfactory cure is obtained for a period of about two weeks after mixing. The curability can be restored by adding additional catalyst, alkoxysilane or both.